The magnetic characteristics of rubber-base bonded magnets that utilize rubber-base resin as binder are strongly affected by not only the characteristics of the magnetic powder used but also the filling rate of the magnetic powder in the rubber-base binder. Although the filling rate of magnetic powder in the rubber-base binder is affected by various factors, including the particle diameter and particle size distribution of the magnetic powder, the shape and surface configuration of the particles and the type of rubber-base binder, what is basically essential is for the magnetic powder to have high compatibility with the rubber-base binder without altering its inherent properties. In this specification, such property of a magnetic powder not degrading the intrinsic properties of a rubber-base binder is referred to as “magnetic powder non-reactivity” and the compatibility thereof with the rubber-base binder is referred to as “magnetic powder affinity.”
When the magnetic powder non-reactivity and affinity are poor, viscosity becomes high and fluidity low at the time of kneading with the rubber-base binder or at the time of forming a compound. As a result, the magnetic particles are subjected to mechanical stress. The strain that arises in the magnetic particles owing to the application of mechanical stress degrades the coercive force of the magnetic powder.
The non-reactivity and affinity of a ferrite magnetic powder can be assessed by, for example, measuring the viscosity and/or shear stress of the compound. The non-reactivity and affinity (compatibility) with respect to the resin can be considered to be better in proportion as the viscosity and/or shear stress is smaller.
JP 2001-160506 A('506) teaches that a ferrite magnetic powder having a carbon content of 0.010-0.040 wt % and a pH of between 6 and less than 9 can be obtained by pulverizing a calcinated product composed of ferrite, annealing the result to obtain a ferrite magnetic powder having a pH of not less than 9, suspending the powder in water and blowing carbon dioxide gas into the suspension. The product is said to be excellent in non-reactivity and affinity with respect to resin-base binder. Reference '506 further teaches that in neutralization by adding a mineral acid instead of blowing in carbon dioxide gas, coherence occurring in the dried product makes necessary a considerably strong crushing process and, in such case, total prevention of occurrence of internal stress is difficult.
Japanese Patent No. 3294507 teaches that a ferrite magnetic powder having a carbon content of 0.015-0.080 wt % and a pH of between 7 and less than 10 can be obtained by bringing annealed ferrite magnetic powder into contact with a CO2 source under stirring. The product is said to be excellent in non-reactivity and affinity with respect to resin-base binder.